Microporous membranes can be used as battery separator film (“BSF”) in, e.g., lithium ion batteries. Large-capacity batteries such as those that can be used to power electric vehicles and hybrid electric vehicles could be improved by increasing the BSFs meltdown temperature and puncture strength without significantly decreasing other important membrane properties such as porosity, permeability, and thermal stability (heat shrinkage). Increasing strength is important because it reduces the risk of internal short circuits resulting from BSF failure.
Monolayer BSFs comprising (i) polymethylpentene and polyethylene and (ii) polymethylpentene and polypropylene have meltdown temperatures ≧200° C., but these films have a lower pin puncture strength than BSFs comprising polyethylene and/or polypropylene.